Many communications systems in use today (i.e. LTE and DVB) are based on orthogonal frequency division multiplexing (OFDM) modulation. While OFDM has the advantage of tolerance to delay spread and is spectrally efficient, it does so at the expense of a high peak-to-average power ratio (PAPR). The high PAPR of OFDM results in low average RF power amplifier (PA) efficiency.
Historically, RF power amplifiers (RF PAs) have been linear devices. With the advent of high peak-to-average power ratio (PAPR) communications and broadcast standards like LTE and DVB, the RF PA is forced to operate in a low efficiency region of its transfer function, if the signal is to be amplified with high fidelity.
A number of amplifier architectures exist to increase PA efficiency under these conditions. For instance, load modulation architectures such as Doherty exists, which uses a circuitry having two amplifiers that modulate each other's load depending on the required output power. Amplifier circuitry with envelope modulators for signal envelope tracking has also been used. Another type of circuit for increasing RF PA efficiency is the outphasing amplifier. Such a circuit uses an isolating combiner with saturated amplifier stages to achieve high linearity and good peak efficiency. Outphasing amplifier circuits using a non-isolating combiner to enable load modulation also exist. Although such amplifier architectures in theory do result in some efficiency increase, in use the improvements are limited. There still exists the problem low average efficiency when amplifying contemporary communications signals like LTE.
Recently, hybrid schemes combining an outphasing amplifier with an envelope tracking power supply have been studied. However, existing hybrid approaches proposed are complicated and involve the switching of supply voltages/gain to the RF PA, which is likely to introduce noise into the system.
There therefore exists a need for to overcome the limited back-off efficiency of the classic outphasing architectures, and the complicated circuits of the noisy hybrid architectures for RF power amplifier applications.